1. Field of the Invention
The present invention relates to a so-called serial scan type ink jet printing apparatus.
2. Description of the Related Art
Known ink jet printing apparatuses print images by using an ink tank that accommodates pigment ink and a print head that ejects the pigment ink fed from the ink tank. The pigment ink tends to have its pigment component precipitated to the bottom of the ink tank. The precipitated pigment component may produce an area with a higher ink concentration and an area with a lower ink concentration in the ink tank.
Japanese Patent Laid-Open No. 2005-066520 describes an ink jet printing apparatus that performs printing operations by moving a carriage equipped with a print head and an ink tank. The printing apparatus utilizes the inertia force of the carriage during a printing operation to allow a stirrer in the ink tank to stir the ink. According to Japanese Patent Laid-Open No. 2005-066520, the stirrer is integrated with a case of the ink tank by molding. The stirrer extends in the case of the ink tank so as to hang down from its ceiling to bottom. The stirrer has a cylindrical weight formed at its lower end. The stirrer stirs the ink in the ink tank by swinging around its root on the ceiling of the case in the same direction in which the carriage moves, under an inertia force resulting from operations of the carriage including acceleration, stoppage, and reversal.
Japanese Patent Laid-Open No. 2005-066520 describes a stirrer that is freely movable on an inner bottom surface of an ink tank instead of being fixed to its case. The stirrer stirs the ink in the ink tank by moving on the bottom surface of the ink tank under an inertia force resulting from operations of the carriage including acceleration, stoppage, and reversal.
Japanese Patent Laid-Open No. 2004-216761 discloses a stirring mechanism comprising a shaft-like weight that swings around a swinging central shaft in a lateral direction under an inertia force resulting from movement of the carriage and a plurality of fins that swing in the lateral direction integrally with the shaft-like weight. The plurality of fins are juxtaposed in the height direction of an ink tank to evenly stir the ink in the ink tank from its upper layer to lower layer.
A known method for stirring a pigment component precipitated to the bottom of an ink tank provides a moving object or a space in the ink tank to stir the ink utilizing operations of the carriage in the ink jet printing apparatus.
Japanese Patent Laid-Open No. 09-309212 and U.S. Pat. No. 6,062,682 describe a configuration that moves a carriage to stir ink, upon receiving a signal for powering on the printing apparatus, a print signal, or a cleaning signal. A predetermined printing operation and an operation of cleaning a print head are performed after a stirring operation. These documents also describe a configuration that detects the time for which a printing operation is stopped. If the printing operation is stopped for at least a predetermined time (hereinafter referred to as a “stirring operation wait time”)), a stirring operation is performed to prevent the precipitation of a pigment component.
As described in Japanese Patent Laid-Open No. 09-309212, if a printing operation is stopped for at least the predetermined time (stirring operation wait time) to keep the ink tank stationary for at least a predetermined time, the ink needs to be sufficiently stirred before the subsequent printing operation or cleaning operation. This is required to achieve favorable printing results and to prevent a pigment component from being fixed in an ink channel.
However, the use period of the ink jet printing apparatus varies; the ink jet printing apparatus is often consecutively operated in very short periods. Obviously, a short printing operation may be repeated during the predetermined stirring operation wait time. Thus, with a simple scheme of waiting for at least the stirring operation wait time to pass before a stirring operation is performed, no stirring operation may be performed over a long period. In this case, the precipitation of the pigment component may progress to cause inappropriate printing in spite of the small amount of pigment component precipitated.
To prevent this problem, a very small value may be set for the stirring operation wait time, after which a stirring operation or a cleaning operation is performed. However, this increases the frequency at which a stirring operation or a cleaning operation is performed, possibly significantly reducing printing speed.
Further, the cleaning operation may include an operation (suction recovering operation) of sucking and discharging ink not contributing to image printing from nozzles in order to avoid blockage in nozzles in the print head and to discharge bubbles mixed into the ink channel. In this case, a suction recovering operation may be performed a predetermined time after the execution of the last suction recovering operation. The suction recovering operation is effective for avoiding the adverse effect of precipitation of the pigment component in areas in the nozzles or ink channel which are not subjected to the stirring operation. However, it is very difficult to discharge all of the pigment component precipitated in the ink tank simply by means of the suction recovering operation.
Further, with a high carriage moving speed and a large printing amount for a printing operation, the ink in the ink tank on the carriage can be expected to be stirred during the printing operation. However, owing to the variation of the printing speed and amount, the ink in the ink tank is not always stirred.